How to send a command APDU to a HCE device?

Question

The AID of my app is F239856324897348 and I have constructed a SelectAID APDU for it. Now how do I actually send it to the receiving Android device which is using host card emulation.

I have a created my HCE service to respond with a response APDU as in this thread: How to define an APDU for STORE DATA for Host Card Emulation?

public static byte[] SelectAID = new byte[]{
        (byte) 0xF2, (byte) 0x39, (byte) 0x85, (byte) 0x63,
        (byte) 0x24, (byte) 0x89, (byte) 0x73, (byte) 0x48};

private void commandAPDU(byte[] apdu){
   //where do I go from here...
}

commandAPDU(SelectAID);

Answer 1

The format of APDUs is defined in ISO/IEC 7816-4. A typical SELECT (by AID) command looks like this:

+-----+-----+-----+-----+-----+-------------------------+-----+
| CLA | INS | P1  | P2  | Lc  | DATA                    | Le  |
+-----+-----+-----+-----+-----+-------------------------+-----+
| 00  | A4  | 04  | 00  | XX  | AID                     | 00  |
+-----+-----+-----+-----+-----+-------------------------+-----+

You could create it like this:

private byte[] selectApdu(byte[] aid) {
    byte[] commandApdu = new byte[6 + aid.length];
    commandApdu[0] = (byte)0x00;  // CLA
    commandApdu[1] = (byte)0xA4;  // INS
    commandApdu[2] = (byte)0x04;  // P1
    commandApdu[3] = (byte)0x00;  // P2
    commandApdu[4] = (byte)(aid.length & 0x0FF);       // Lc
    System.arraycopy(aid, 0, commandApdu, 5, aid.length);
    commandApdu[commandApdu.length - 1] = (byte)0x00;  // Le
    return commandApdu;
}

You could then send such APDU commands to a tag/HCE device discovered through the reader-mode API:

public abstract void onTagDiscovered(Tag tag) {
    IsoDep isoDep = IsoDep.get(tag);
    if (isoDep != null) {
        try {
            isoDep.connect();
            byte[] result = isoDep.transceive(selectApdu(SelectAID));
        } except (IOException ex) {
        } finally {
            try {
                isoDep.close();
            } except (Exception ignored) {}
        }
    }
}